The invention concerns an improved strainer, particularly suitable for cleaning fibrous suspensions by separating the foreign bodies and the impurities that contaminate them.
The strainer object of the invention is designed to be used in particular in the papermaking sector, above all for cleaning aqueous suspensions of fibres deriving from recycled paper.
It is known that to remove the contaminants present in the aqueous suspensions of fibres used in the papermaking industry, and in particular in the suspensions obtained from maceration paper, special strainers are used, in which the suspension is filtered by making it pass through baskets provided with filtering surfaces.
The completely filtered suspension that comes out is usually indicated as “accepted” suspension.
It can be easily understood that while the clean suspension flows through the filtering surface the contaminants tend to concentrate on the same filtering surface.
In order to counteract this tendency, a fair percentage of suspension is drained, so that it brings along the contaminants and in such a way as to reduce to the minimum the time during which they remain in contact with the filtering surface, thus preventing them from accumulating thereon.
The suspension containing contaminants mixed with a considerable quantity of fibre constitutes the so-called “rejected” suspension and is processed in correspondence with strainers arranged according to a “cascade” layout, each one of which recovers part of the fibre in the “accepted” suspension outlet and concentrates the contaminants in the “rejected” suspension line.
These strainers are called primary, secondary, tertiary and so on, until the last one, called “final”, from which the contaminants are definitively removed together with a small quantity of fibre.
The strainer chain is usually constituted by three units, which may also be four in the case of considerable production quantities.
Due to the increasingly higher concentration of contaminants and of increasingly longer fibre, the latter being the most sought after and the most difficult to be recovered, present in the suspension during the treatment involving passage from one strainer to the other, these operate with different characteristics in terms of suspension density and of the speed at which the suspension passes through the filtering surface.
Furthermore, due to the concentration of contaminants, the “accepted” suspension lines deriving from each strainer become less and less clean and cannot proceed and be added to the main “accepted” suspension line of the primary strainer.
For this reason the suspension must be re-processed in the immediately preceding strainer and this involves considerable complexity of the system, which must include intermediate containers, stirrers, pumps, tubes, various types of valves, accessories for intermediate dilutions, control instruments and, last but not least, much space and energy.
The drawbacks of these systems are clear and they derive above all from the high costs met by the paper mill for the purchase, installation and maintenance of several strainers.
In order to overcome the drawbacks mentioned above, manufacturers offer the market improved strainers that group into single machines the functions performed by several machines, thus allowing the size of the systems and installation, management and maintenance costs to be reduced.
These strainers are big machines that beside the advantages described above also have a drawback represented by the fact that the filtering elements with which they are provided aren't all in the same operating conditions and therefore tend to get clogged very easily.
Patents are known, which describe different products aiming to solve the problem posed by the clogging of the filtering surfaces of the strainers.
Patent U.S. 2002/0069985 partly solves the above mentioned problem through the implementation of a strainer whose filtering surface comprises two overlapping filtering baskets, wherein the upper basket has the shape of a truncated cone with its tapered part converging upwards.
Two drain pipes for the “rejected” suspension are provided, which are arranged at different axial distances with respect to the bottom of the machine, while the “accepted” suspension is extracted near the bottom, through a delivery pipe positioned directly above the “rejected” suspension outlet pipes.
Patents EP 0 931 875 and WO 02/064884, both registered in the name of the same applicant that files this patent application, solve the problem represented by the clogging of the filtering element, by shaping the rotors according to special profiles and in particular through protruding blades with wing-shaped profile that during rotation generate a vacuum condition in correspondence with the filtering surface, thus facilitating the detachment of the impurities.
U.S. Pat. No. 6,360,897 solves the problem represented by clogging and improves the filtering action by dividing the entire throughput of the suspension to be filtered into two annular chambers that are independent of each other.
U.S. Pat. No. 6,311,850 improves the filtering action by using a rotor whose outer surface is provided with protruding elements, the profile of which varies according to their position with respect to the bottom of the rotor and which therefore act in a differentiated way on the fibrous suspension that becomes increasingly thicker near the bottom.
U.S. Pat. No. 5,318,186 and EP 1 122 358 describe a strainer in which the filtering action is improved through the pre-filtering of the suspension at the strainer inlet.
U.S. Pat. No. 0,139,723 describes a strainer in which the filtering action is improved by using two filtering baskets positioned in series.
Finally, patent WO 94/16141 describes a strainer in which the filtering action is improved by using segregation chambers that separate the large-sized contaminants, for example stones or other macroscopic impurities, before they make contact with the filtering surface of the strainer.
All the strainers described in the mentioned patents achieve the goal to reduce the clogging of the filtering elements, but, like conventional strainers, they must be installed in the processing line after a machine called “high/medium density cleaner”, substantially comprising a centrifugal cleaner that removes most of the heavy contaminants, which are abrasive by nature and therefore reduce the operating life of the filtering surfaces.
Centrifugal cleaners, however, do not have any effect on very small-sized contaminants, which are drawn by the viscous fluid of high/medium density and do not migrate towards the periphery of the vortex due to the centrifugal action generated by the centrifugal cleaner, as is the case, instead, with heavy and large-sized contaminants.
Such light and small-sized contaminants, if they are not intercepted by the filtering surfaces of the screen where they create a damage in any case, are successively removed by centrifugal cleaners that operate where the suspension density is sufficiently low, for example in the flotation area.
The high/medium density centrifugal cleaners remove neither light contaminants, which usually do not create damages due to their abrasive action, nor contaminants with filaments, whose specific weights don't usually differ much from those of water and cellulose fibre.
They are strips of fabric, strips of plastic, strips and knots of paper chemically treated as water-repellent that during the kneading phase hasn't been sufficiently disaggregated.
The damage due to the presence of such contaminants with filaments inside the strainer is represented by their tendency to overlap and accumulate wherever there are protruding surfaces, typically on the edges of the wing-shaped surfaces of the rotors or of the connection elements that connect the wing-shaped surfaces to the body of the rotor.
The presence of these contaminants with filaments reduces the performance levels of the strainer, in terms of both throughput and production of secondary contaminants originating from the fragmentation due to abrasion against the filtering surfaces that are usually rough in order to ensure proper microturbulence near the precisely sized openings.
In addition to their positive characteristics, concerning:                capacity to filter large throughputs of fibrous suspension to reduce the number of machines to be installed in the system;        capacity to reduce to the minimum the throughput of “rejected” suspension, in such a way as to limit its processing to a single final unit;        capacity to limit and if possible prevent the clogging of the various filtering surfaces,        the strainers mentioned above, however, have the drawback that they don't offer the following advantages, if not in a limited and in any case non-selective manner:        capacity to intercept and separate abrasive contaminants, usually heavy, even if small-sized, before the suspension makes contact with the filtering surface;        capacity to intercept and separate fragile contaminants, usually light, even if small-sized, before the suspension makes contact with the filtering surface, in order to prevent their fragmentation, being in fact well known that the energetic stirring action produced by the strainer rotor in order to keep the filtering surface clean produces the so-called secondary contaminants;        capacity to intercept and separate the contaminants with filaments, preventing their stagnation in the area included between the rotor and the filtering surface.        
The drawback of the known strainers mentioned above is constituted by the fact that none of them offers all the listed characteristics together.
The present invention aims to implement an improved strainer with rotor having all the characteristics mentioned above together, while these characteristics can be found only separately in the mechanical strainers mentioned above.